Fibroblast studies documenting a case of peroxisomal 2-methylacyl-CoA racemase deficiency: possible link between racemase deficiency and malabsorption and vitamin K deficiency

Alpha-methyl acetyl-coA racemase deficiency. Magnetic resonance imaging findings of three patients with encephalopathy, epilepsy, and stroke-like episodes

Alpha-methyl acyl-CoA racemase deficiency (AMACRD) is a rare peroxisomal disorder that results in the accumulation of pristanic acid and 16 cases have been reported in the literature. Here, we present three additional patients, two confirmed by genomic study and one suspected. Three siblings who were born to healthy unrelated parents developed recurrent episodes of encephalopathy, seizures, and behavioral disturbances. In all 3, brain MRI showed lesions in the thalami, cerebral peduncles, and mesencephalic tegmentum, as well as brain volume loss. In addition, one patient had a chronic hemispheric infarct and an acute contralateral infarct, and another had a subacute infarct involving multiple vascular territories without abnormalities on MR angiography.

Autoantibody Positivity in Two Bahraini Siblings With a Novel Alpha-Methylacyl-CoA Racemase Mutation

Bile acid synthesis disorders (BASD) are a group of rare autosomal recessive disorders. Of the nine different versions, BASD type 4 is characterized by a gene mutation in alpha-methylacyl-CoA racemase (AMACR), which is located on chromosome 5p13. These disorders generally present with a normal gamma-glutamyl transferase with cholestasis, absence of pruritis, and malabsorption of fat, which can lead to fat-soluble vitamin deficiencies. In adulthood, patients usually develop neurological sequelae. Initial testing can be done through the measurement of urine metabolites; however, confirmation of the diagnosis is achieved through whole exome sequencing. Treatment involves supplementation of oral cholic acid and modification of diet. Only 23 patients with this disease have been described. Here, we report two cases of siblings from a family in Bahrain with a novel AMACR mutation and a unique association with autoimmune antibodies alongside a literature review.

Late onset AMACR deficiency with metabolic stroke-like episodes and seizures

Alpha-methylacyl-CoA racemase (AMACR) deficiency is a rare peroxisomal disorder causing pristanic acid accumulation. Only 16 cases have been described so far. A female in her seventh decade presented with episodes of dysphasia, headache and sensory disturbance inconsistent with migraine, epilepsy or transient ischaemic attack. An MRI demonstrated unusual changes in the pons, red nuclei, thalami and white matter. Mitochondrial disease was suspected but detailed testing was negative. After eight years of symptoms, she developed a febrile encephalopathy with hemispheric dysfunction, focal convulsive seizures and coma. Her condition stabilised after one month. Lacosamide was continued for seizure prevention. The diagnosis remained elusive until whole genome sequencing revealed AMACR deficiency. Pristanic acid levels were highly elevated and dietary modification was recommended. Genetic peroxisomal disorders can present in older age; our patient is the oldest in the AMACR deficiency literature. Novel features in our case include central apnoea, dystonia and rapid eye movement behaviour disorder.

Diagnosis and treatment of an inborn error of bile acid synthesis type 4: A case report

BACKGROUND

Inborn error of bile acid synthesis type 4 is a peroxisomal disease with impaired bile acid synthesis caused by a-methylacyl-CoA racemase (AMACR) gene mutation. The disease is usually found in children with mild to severe liver disease, cholestasis and poor fat-soluble vitamin absorption. At present, there is no report of inborn errors of bile acid synthesis type 4 in adults with liver disease and poor fat-soluble vitamin absorption.

CASE SUMMARY

A 71-year-old man was hospitalized in our department for recurrent liver dysfunction. The clinical manifestations were chronic liver disease and yellow skin and sclera. Serum transaminase, bilirubin and bile acid were abnormally increased; and fat-soluble vitamins decreased. Liver cirrhosis and ascites were diagnosed by computed tomography. The patient had poor coagulation function and ascites and did not undergo liver puncture. Genetic testing showed AMACR gene missense mutation. The patient was diagnosed with inborn error of bile acid synthesis type 4. He was treated with ursodeoxycholic acid, liver protection and vitamin supplementation, and jaundice of the skin and sclera was reduced. The indicators of liver function and the quality of life were significantly improved.

CONCLUSION

When adults have recurrent liver function abnormalities, physicians should be alert to genetic diseases and provide timely treatment.

Episodic hyperCKaemia may be a feature of α-methylacyl-coenzyme A racemase deficiency

α-methylacyl-CoA racemase (AMACR) deficiency is a rare disorder, affecting peroxisomal metabolism of pristanic acid, with ten published adult cases. We describe an AMACR deficiency case with a clinical presentation dominated by episodic hyperCKaemia, suggesting that myopathic features of AMACR should be considered.

Slc25a17 Gene Trapped Mice: PMP34 Plays a Role in the Peroxisomal Degradation of Phytanic and Pristanic Acid

Mice lacking PMP34, a peroxisomal membrane transporter encoded by Slc25a17, did not manifest any obvious phenotype on a Swiss Webster genetic background, even with various treatments designed to unmask impaired peroxisomal functioning. Peroxisomal α- and β-oxidation rates in PMP34 deficient fibroblasts or liver slices were not or only modestly affected and in bile, no abnormal bile acid intermediates were detected. Peroxisomal content of cofactors like CoA, ATP, NAD+, thiamine-pyrophosphate and pyridoxal-phosphate, based on direct or indirect data, appeared normal as were tissue plasmalogen and very long chain fatty acid levels. However, upon dietary phytol administration, the knockout mice displayed hepatomegaly, liver inflammation, and an induction of peroxisomal enzymes. This phenotype was partially mediated by PPARα. Hepatic triacylglycerols and cholesterylesters were elevated and both phytanic acid and pristanic acid accumulated in the liver lipids, in females to higher extent than in males. In addition, pristanic acid degradation products were detected, as wells as the CoA-esters of all these branched fatty acids. Hence, PMP34 is important for the degradation of phytanic/pristanic acid and/or export of their metabolites. Whether this is caused by a shortage of peroxisomal CoA affecting the intraperoxisomal formation of pristanoyl-CoA (and perhaps of phytanoyl-CoA), or the SCPx-catalyzed thiolytic cleavage during pristanic acid β-oxidation, could not be proven in this model, but the phytol-derived acyl-CoA profile is compatible with the latter possibility. On the other hand, the normal functioning of other peroxisomal pathways, and especially bile acid formation, seems to exclude severe transport problems or a shortage of CoA, and other cofactors like FAD, NAD(P)+, TPP. Based on our findings, PMP34 deficiency in humans is unlikely to be a life threatening condition but could cause elevated phytanic/pristanic acid levels in older adults.

Bile acid analysis in human disorders of bile acid biosynthesis

Bile acids facilitate the absorption of lipids in the gut, but are also needed to maintain cholesterol homeostasis, induce bile flow, excrete toxic substances and regulate energy metabolism by acting as signaling molecules. Bile acid biosynthesis is a complex process distributed across many cellular organelles and requires at least 17 enzymes in addition to different metabolite transport proteins to synthesize the two primary bile acids, cholic acid and chenodeoxycholic acid. Disorders of bile acid synthesis can present from the neonatal period to adulthood and have very diverse clinical symptoms ranging from cholestatic liver disease to neuropsychiatric symptoms and spastic paraplegias. This review describes the different bile acid synthesis pathways followed by a summary of the current knowledge on hereditary disorders of human bile acid biosynthesis with a special focus on diagnostic bile acid profiling using mass spectrometry.

Safety of long-term restrictive diets for peroxisomal disorders: vitamin and trace element status of patients treated for Adult Refsum Disease

BACKGROUND

Adult Refsum's Disease (ARD) is caused by defects in the pathway for alpha-oxidation of phytanic acid (PA). Treatment involves restricting the dietary intake of phytanic acid by reducing the intake of dairy-derived fat. The adequacy of micronutrient intake in patients with ARD is unknown.

METHODS

Patients established on the Chelsea low-PA diet had general diet macronutrients, vitamins and trace elements assessed using 7-day-weighed intakes and serial 24-h recalls. Intakes were compared with biochemical assessments of nutritional status for haematinics (ferritin), trace elements (copper, zinc, iron, selenium), water- (vitamin B6 , B12 and folate) and fat-soluble vitamins (A, D, E and K).

RESULTS

Eleven subjects (four women, seven men) were studied. Body mass index was 27 ± 5 kg/m(2) (range 19-38). All subjects had high sodium intakes (range 1873-4828 mg). Fat-soluble vitamin insufficiencies occurred in some individuals (vitamin A, n = 2; vitamin D, n = 6; vitamin E, n = 3; vitamin K, n = 10) but were not coincident. Vitamin B6 levels were normal or elevated (n = 6). Folate and 5-methyltetrahydrofolate concentrations were normal. Metabolic vitamin B12 insufficiency was suspected in four subjects based on elevated methylmalonic acid concentrations. Low copper and selenium intakes were noted in some subjects (n = 7, n = 2) but plasma levels were adequate. Iron, ferritin and zinc intakes and concentrations were normal.

CONCLUSION

Subjects with ARD can be safely managed on the Chelsea low PA without routine micronutrient supplementation. Sodium intake should be monitored and reduced. Periodic nutritional screening may be necessary for fat-soluble vitamins, vitamin B12 , copper or selenium.

The clinical spectrum of inherited diseases involved in the synthesis and remodeling of complex lipids. A tentative overview

Over one hundred diseases related to inherited defects of complex lipids synthesis and remodeling are now reported. Most of them were described within the last 5 years. New descriptions and phenotypes are expanding rapidly. While the associated clinical phenotype is currently difficult to outline, with only a few patients identified, it appears that all organs and systems may be affected. The main clinical presentations can be divided into (1) Diseases affecting the central and peripheral nervous system. Complex lipid synthesis disorders produce prominent motor manifestations due to upper and/or lower motoneuron degeneration. Motor signs are often complex, associated with other neurological and extra-neurological signs. Three neurological phenotypes, spastic paraparesis, neurodegeneration with brain iron accumulation and peripheral neuropathies, deserve special attention. Many apparently well clinically defined syndromes are not distinct entities, but rather clusters on a continuous spectrum, like for the PNPLA6-associated diseases, extending from Boucher-Neuhauser syndrome via Gordon Holmes syndrome to spastic ataxia and pure hereditary spastic paraplegia; (2) Muscular/cardiac presentations; (3) Skin symptoms mostly represented by syndromic (neurocutaneous) and non syndromic ichthyosis; (4) Retinal dystrophies with syndromic and non syndromic retinitis pigmentosa, Leber congenital amaurosis, cone rod dystrophy, Stargardt disease; (5) Congenital bone dysplasia and segmental overgrowth disorders with congenital lipomatosis; (6) Liver presentations characterized mainly by transient neonatal cholestatic jaundice and non alcoholic liver steatosis with hypertriglyceridemia; and (7) Renal and immune presentations. Lipidomics and molecular functional studies could help to elucidate the mechanism(s) of dominant versus recessive inheritance observed for the same gene in a growing number of these disorders.

MRI characterisation of adult onset alpha-methylacyl-coA racemase deficiency diagnosed by exome sequencing

BACKGROUND

Correct diagnosis is pivotal to understand and treat neurological disease. Herein, we report the diagnostic work-up utilizing exome sequencing and the characterization of clinical features and brain MRI in two siblings with a complex, adult-onset phenotype; including peripheral neuropathy, epilepsy, relapsing encephalopathy, bilateral thalamic lesions, type 2 diabetes mellitus, cataract, pigmentary retinopathy and tremor.

METHODS

We applied clinical and genealogical investigations, homozygosity mapping and exome sequencing to establish the diagnosis and MRI to characterize the cerebral lesions.

RESULTS

A recessive genetic defect was suspected in two siblings of healthy, but consanguineous parents. Homozygosity mapping revealed three shared homozygous regions and exome sequencing, revealed a novel homozygous c.367 G>A [p.Asp123Asn] mutation in the α-methylacyl-coA racemase (AMACR) gene in both patients. The genetic diagnosis of α-methylacyl-coA racemase deficiency was confirmed by demonstrating markedly increased pristanic acid levels in blood (169 μmol/L, normal <1.5 μmol/L). MRI studies showed characteristic degeneration of cerebellar afferents and efferents, including the dentatothalamic tract and thalamic lesions in both patients.

CONCLUSIONS

Metabolic diseases presenting late are diagnostically challenging. We show that appropriately applied, homozygosity mapping and exome sequencing can be decisive for establishing diagnoses such as late onset α-methylacyl-coA racemase deficiency, an autosomal recessive peroxisomal disorder with accumulation of pristanic acid. Our study also highlights radiological features that may assist in diagnosis. Early diagnosis is important as patients with this disorder may benefit from restricted dietary phytanic and pristanic acid intake.

Biochemistry and genetics of inherited disorders of peroxisomal fatty acid metabolism

In humans, peroxisomes harbor a complex set of enzymes acting on various lipophilic carboxylic acids, organized in two basic pathways, alpha-oxidation and beta-oxidation; the latter pathway can also handle omega-oxidized compounds. Some oxidation products are crucial to human health (primary bile acids and polyunsaturated FAs), whereas other substrates have to be degraded in order to avoid neuropathology at a later age (very long-chain FAs and xenobiotic phytanic acid and pristanic acid). Whereas total absence of peroxisomes is lethal, single peroxisomal protein deficiencies can present with a mild or severe phenotype and are more informative to understand the pathogenic factors. The currently known single protein deficiencies equal about one-fourth of the number of proteins involved in peroxisomal FA metabolism. The biochemical properties of these proteins are highlighted, followed by an overview of the known diseases.

Relapsing encephalopathy in a patient with α-methylacyl-CoA racemase deficiency

α-Methylacyl-CoA racemase (AMACR) deficiency is a rare disorder of fatty acid metabolism which has recently been described in three adult cases. We have identified a further patient with clinical features of a relapsing encephalopathy, seizures and cognitive decline over a 40 year period. Biochemical studies revealed grossly elevated plasma pristanic acid levels, and a deficiency of AMACR in skin fibroblasts. Sequence analysis of AMACR cDNA identified a homozygous point mutation (c154T>C). This case adds to the phenotypic variation seen in this peroxisomal disorder and highlights the importance of screening for plasma pristanic acid levels in patients with unexplained relapsing encephalopathies.

Cloning and characterization of alpha-methylacyl coenzyme A racemase from Gordonia polyisoprenivorans VH2

The mcr gene of Gordonia polyisoprenivorans VH2 is not clustered with genes required for rubber degradation. Its disruption by insertion of a kanamycin resistance cassette impaired growth on methyl-branched isoprenoids but not on linear hydrocarbons. Intact mcr from this bacterium or from Nocardia farcinica IFM 10152 complemented the mutant. Reverse transcription analysis showed similar mcr(VH2) expression results during cultivation with poly(cis-1,4-isoprene) and propionate. Additional genes coding for a putative cytochrome P450 monooxygenase and a short-chain dehydrogenase/reductase involved in beta-oxidation and poly(cis-1,4-isoprene) degradation were also characterized.

Mechanisms of disease: Inborn errors of bile acid synthesis

Inborn errors of bile acid synthesis are rare genetic disorders that can present as neonatal cholestasis, neurologic disease or fat-soluble-vitamin deficiencies. There are nine known defects of bile acid synthesis, including oxysterol 7alpha-hydroxylase deficiency, Delta(4)-3-oxosteroid-5beta-reductase deficiency, 3beta-hydroxy-Delta(5)-C(27)-steroid dehydrogenase deficiency, cerebrotendinous xanthomatosis (also known as sterol 27-hydroxylase deficiency), alpha-methylacyl-CoA racemase deficiency, and Zellweger syndrome (also known as cerebrohepatorenal syndrome). These diseases are characterized by a failure to produce normal bile acids and an accumulation of unusual bile acids and bile acid intermediaries. Individuals with inborn errors of bile acid synthesis generally present with the hallmark features of normal or low serum bile acid concentrations, normal gamma-glutamyl transpeptidase concentrations and the absence of pruritus. Failure to diagnose any of these conditions can result in liver failure or progressive chronic liver disease. If recognized early, many patients can have a remarkable clinical response to oral bile acid therapy.

Alpha-methylacyl-CoA racemase--an 'obscure' metabolic enzyme takes centre stage

Branched-chain lipids are important components of the human diet and are used as drug molecules, e.g. ibuprofen. Owing to the presence of methyl groups on their carbon chains, they cannot be metabolized in mitochondria, and instead are processed and degraded in peroxisomes. Several different oxidative degradation pathways for these lipids are known, including alpha-oxidation, beta-oxidation, and omega-oxidation. Dietary branched-chain lipids (especially phytanic acid) have attracted much attention in recent years, due to their link with prostate, breast, colon and other cancers as well as their role in neurological disease. A central role in all the metabolic pathways is played by alpha-methylacyl-CoA racemase (AMACR), which regulates metabolism of these lipids and drugs. AMACR catalyses the chiral inversion of a diverse number of 2-methyl acids (as their CoA esters), and regulates the entry of branched-chain lipids into the peroxisomal and mitochondrial beta-oxidation pathways. This review brings together advances in the different disciplines, and considers new research in both the metabolism of branched-chain lipids and their role in cancer, with particular emphasis on the crucial role played by AMACR. These recent advances enable new preventative and treatment strategies for cancer.

Peroxisomal disorders: the single peroxisomal enzyme deficiencies

Peroxisomal disorders are a group of inherited diseases in man in which either peroxisome biogenesis or one or more peroxisomal functions are impaired. The peroxisomal disorders identified to date are usually classified in two groups including: (1) the disorders of peroxisome biogenesis, and (2) the single peroxisomal enzyme deficiencies. This review is focused on the second group of disorders, which currently includes ten different diseases in which the mutant gene affects a protein involved in one of the following peroxisomal functions: (1) ether phospholipid (plasmalogen) biosynthesis; (2) fatty acid beta-oxidation; (3) peroxisomal alpha-oxidation; (4) glyoxylate detoxification, and (5) H2O2 metabolism.

α-Methylacyl-CoA Racemase from Mycobacterium tuberculosis

Alpha-methylacyl-CoA racemase (Amacr) catalyzes the racemization of alpha-methyl-branched CoA esters. Sequence comparisons have shown that this enzyme is a member of the family III CoA transferases. The mammalian Amacr is involved in bile acid synthesis and branched-chain fatty acid degradation. In human, mutated variants of Amacr have been shown to be associated with disease states. Amino acid sequence alignment of Amacrs and its homologues from various species revealed 26 conserved protic residues, assumed to be potential candidates as catalytic residues. Amacr from Mycobacterium tuberculosis (MCR) was taken as a representative of the racemases. To determine their importance for efficient catalysis, each of these 26 protic residues of MCR was mutated into an alanine, respectively, and the mutated variants were overexpressed in Escherichia coli. It was found that four variants (R91A, H126A, D156A, and E241A) were properly folded but had much decreased catalytic efficiency. Apparently, Arg91, His126, Asp156, and Glu241 are important catalytic residues of MCR. The importance of these residues for catalysis can be rationalized by the 1.8 A resolution crystal structure of MCR, which shows that the catalytic site is at the interface between the large and small domain of two different subunits of the dimeric enzyme. This crystal structure is the first structure of a complete enzyme of the bile acid synthesis pathway. It shows that MCR has unique structural features, not seen in the structures of the sequence related formyl-CoA transferases, suggesting that the family III CoA transferases can be subdivided in at least two classes, being racemases and CoA transferases.

Role of alpha-methylacyl coenzyme A racemase in the degradation of methyl-branched alkanes by Mycobacterium sp. strain P101

A new isolate, Mycobacterium sp. strain P101, is capable of growth on methyl-branched alkanes (pristane, phytane, and squalane). Among ca. 10,000 Tn5-derived mutants, we characterized 2 mutants defective in growth on pristane or n-hexadecane. A single copy of Tn5 was found to be inserted into the coding region of mcr (alpha-methylacyl coenzyme A [alpha-methylacyl-CoA] racemase gene) in mutant P1 and into the coding region of mls (malate synthase gene) in mutant H1. Mutant P1 could not grow on methyl-branched alkanes. The recombinant Mcr produced in Escherichia coli was confirmed to catalyze racemization of (R)-2-methylpentadecanoyl-CoA, with a specific activity of 0.21 micromol . min(-1) . mg of protein(-1). Real-time quantitative reverse transcriptase PCR analyses indicated that mcr gene expression was enhanced by the methyl-branched alkanes pristane and squalane. Mutant P1 used (S)-2-methylbutyric acid for growth but did not use the racemic compound, and growth on n-hexadecane was not inhibited by pristane. These results suggested that the oxidation of the methyl-branched alkanoic acid is inhibited by the (R) isomer, although the (R) isomer was not toxic during growth on n-hexadecane. Based on these results, Mcr is suggested to play a critical role in beta-oxidation of methyl-branched alkanes in Mycobacterium. On the other hand, mutant H1 could not grow on n-hexadecane, but it partially retained the ability to grow on pristane. The reduced growth of mutant H1 on pristane suggests that propionyl-CoA is available for cell propagation through the 2-methyl citric acid cycle, since propionyl-CoA is produced through beta-oxidation of pristane.

Phenotypic variability (heterogeneity) of peroxisomal disorders

Peroxisomes perform a multitude of biosynthetic and catabolic functions, many of which are related to lipid metabolism. Peroxisomal disorders result either from deficiency of a single peroxisomal enzyme or protein, or from a defect in the complex mechanism of peroxisomal biogenesis, resulting in deficiency of several or multiple peroxisomal functions. These can be assessed by a battery of biochemical assays, enabling a biochemical phenotype to be defined that is specific and diagnostic for each of the peroxisomal disorders. Some peroxisomal disorders have unique and specific clinical phenotypes, which may be diagnostic. Others share patterns of clinical abnormalities (particularly neurological dysfunction, craniofacial dysmorphism, skeletal defects, sensory deafness, retinopathy) consistent with defined clinical phenotypes, but with considerable overlap and heterogeneity. To a certain extent, the clinical features of a particular disorder reflect the accumulation or deficiency of specific metabolites. Thus, the same clinical phenotypes may be caused by both single enzyme defects and PBDs. Furthermore, the same defect may present with different clinical phenotypes. In general, the severity of the clinical phenotype correlates with the degree of biochemical dysfunction. The clinical heterogeneity of peroxisomal disorders constitutes a diagnostic challenge demanding a high index of suspicion on the clinician's part.

Bile acid synthetic defects and liver disease: a comprehensive review

Bile acid synthetic defects (BASD), uncommon genetic disorders that are responsible for approximately 2% of persistent cholestasis in infants, are reviewed with emphasis on morphology of associated liver disease. The associated liver diseases may be life threatening, and are treatable, usually by replacement of deficient primary bile acids. Specific diagnosis is made by analysis of body fluids (bile, blood, and urine) using fast atom bombardment-mass spectroscopy (FAB-MS) and gas chromatography-mass spectroscopy (GC-MS). Inborn errors have been demonstrated for four single enzymes involved in modification of the sterol nucleus and in five steps in modification of the side-chain to form cholic and chenodeoxycholic acids, the primary bile acids. With few exceptions, BASD cause liver diseases that vary from severe to mild depending on the defect. In three of four known defects of sterol nucleus modification, liver disease is progressive. Progression of liver disease is most rapid when the defect results in accumulation of toxic monohydroxy and unsaturated oxo-bile acids. Liver disease may be transient, delayed in onset and mild. Reduced bile flow caused by atypical bile acids contributes to cholestasis and may be the dominant factor in defects of side-chain synthesis, peroxisomal abiogenesis and S-L-O syndrome. Pathological findings may include intralobular cholestasis with giant cell transformation, prevalence of necrotic hepatocytes including giant cell forms, and hepatitic injury confined to the portal limiting plate where the smallest bile ductules may be injured and where fibrosis typically develops. Interlobular bile ducts are usually spared. Ultrastructure of liver reveals nonspecific changes with the possible exception of unusual canalicular morphology in some defects. The course of BASD may be modified by replacement of deficient primary bile acids, which produces beneficial feedback inhibition of abnormal bile acid production and enhances choluresis. Giant cell transformation is present in all symptomatic infants with BASD and seems to have a more consistent association with BASD than with the many other liver diseases in infants where it occurs. We hypothesize that immature hepatocytes of infants may fuse to form multinucleate hepatocytes whenever atypical or toxic bile acids are present and the pool of normal bile acids is critically reduced.

A mouse model for α-methylacyl-CoA racemase deficiency: adjustment of bile acid synthesis and intolerance to dietary methyl-branched lipids

alpha-Methylacyl-CoA racemase (Amacr) deficiency in humans leads to sensory motor neuronal and liver abnormalities. The disorder is recessively inherited and caused by mutations in the AMACR gene, which encodes Amacr, an enzyme presumed to be essential for bile acid synthesis and to participate in the degradation of methyl-branched fatty acids. To generate a model to study the pathophysiology in Amacr deficiency we inactivated the mouse Amacr gene. As per human Amacr deficiency, the Amacr(-/-) mice showed accumulation (44-fold) of C27 bile acid precursors and decreased (over 50%) primary (C24) bile acids in bile, serum and liver, however the Amacr(-/-) mice were clinically symptomless. Real-time quantitative PCR analysis showed that, among other responses, the level of mRNA for peroxisomal multifunctional enzyme type 1 (pMFE-1) was increased 3-fold in Amacr(-/-) mice. This enzyme can be placed, together with CYP3A11 and CYP46A1, to make an Amacr-independent pathway for the generation of C24 bile acids. Exposure of Amacr(-/-) mice to a diet supplemented with phytol, a source for branched-chain fatty acids, triggered the development of a disease state with liver manifestations, redefining the physiological significance of Amacr. Amacr is indispensable for the detoxification of dietary methyl-branched lipids and, although it contributes normally to bile acid synthesis from cholesterol, the putative pMFE-1-mediated cholesterol degradation can provide for generation of bile acids, allowing survival without Amacr. Based upon our mouse model, we propose elimination of phytol from the diet of patients suffering from Amacr deficiency.

Diagnosis of inherited disorders of liver metabolism

Diagnosis of the metabolic disorder responsible for liver disease can sometimes be straightforward but it can also present a major challenge, particularly if the liver is sufficiently damaged to produce secondary biochemical abnormalities such as galactosuria, hypoglycaemia with hypoketonaemia, or excretion of 3-oxo-delta4 bile acids. It is important to consider the age of the patient, the nature of the liver disease, any extrahepatic clinical features, the imaging and the first-line laboratory tests when prioritizing diagnostic investigations. This article gives some examples of diagnoses made in our unit for patients with liver disease presenting in utero, in the neonatal period, in infancy and the preschool years, and in the school years. The differential diagnoses that should be considered for different clinical presentations are discussed.

Liver disease caused by failure to racemize trihydroxycholestanoic acid: gene mutation and effect of bile acid therapy

BACKGROUND & AIMS

Inborn errors of bile acid metabolism may present as neonatal cholestasis and fat-soluble vitamin malabsorption or as late onset chronic liver disease. Our aim was to fully characterize a defect in bile acid synthesis in a 2-week-old African-American girl presenting with coagulopathy, vitamin D and E deficiencies, and mild cholestasis and in her sibling, whose liver had been used for orthotopic liver transplantation (OLT).

METHODS

Bile acids were measured by mass spectrometry in urine, bile, serum, and feces of the patient and in urine from the unrelated recipient.

RESULTS

Liver biopsy specimens showed neonatal hepatitis with giant cell transformation and hepatocyte necrosis; peroxisomes were reduced in number. High concentrations of (25R)3alpha,7alpha,12alpha-trihydroxy-5beta-cholestanoic acid in the urine, bile, and serum established a pattern similar to that of Zellweger syndrome and identical to the Alligator mississippiensis. Serum phytanic acid was normal, whereas pristanic acid was markedly elevated. Biochemical, MRI, and neurologic findings were inconsistent with a generalized defect of peroxisomal function and were unique. Analysis of the urine from the recipient of the deceased sibling's liver confirmed the same bile acid synthetic defect. A deficiency in 2-methylacyl-CoA racemase, which is essential for conversion of (25R)THCA to its 25S-isomer, the substrate to initiate peroxisomal beta-oxidation to primary bile acids, was confirmed by DNA analysis revealing a missense mutation (S52P) in the gene encoding this enzyme. Long-term treatment with cholic acid normalized liver enzymes and prevented progression of symptoms.

CONCLUSIONS

This genetic defect further highlights bile acid synthetic defects as a cause of neonatal cholestasis.

Mutations in the sterol 27-hydroxylase gene (CYP27A) cause hepatitis of infancy as well as cerebrotendinous xanthomatosis

Follow-up investigations were undertaken on a previously reported patient who had severe familial giant cell hepatitis in infancy associated with substantially increased urinary excretion of bile alcohol glucuronides. By the age of 11 years, he had developed a profile of cholanoids in plasma and urine that closely resembled the pattern seen in cerebrotendinous xanthomatosis (CTX). Sequencing of the sterol 27-hydroxylase gene (CYP27A) showed that he was homozygous for a deletion (525/526delG) that causes a frameshift and a premature stop codon. This genotype has previously been described in an adult female with classical symptoms of CTX (tendon xanthomata, cataracts and deteriorating cognitive function). A review of past medical histories of a group of patients with CTX revealed that prolonged neonatal cholestatic jaundice was common. The family histories also revealed fetal and neonatal deaths among siblings of patients with CTX. We conclude that defective activity of cholesterol 27-hydroxylase can lead to neonatal cholestatic jaundice ('hepatitis of infancy'), which may be self-limiting. After a latent period, however, progressive accumulation of cholesterol and cholestanol can lead to the xanthomata, neurodegeneration, cataracts and atherosclerosis that are typical of CTX.